Fixing IO.foreach rubyspec, set env variable $_ to nil

spec/tags/1.9/ruby/core/io/foreach_tags.txt

@@ -1,3 +1,2 @@
 fails:IO.foreach when the filename starts with | gets data from a fork when passed -
-fails:IO.foreach sets $_ to nil
 fails:IO.foreach when passed name, object when the object is a Fixnum uses the object as a limit if it is a Fixnum

src/org/jruby/RubyIO.java

@@ -3444,6 +3444,7 @@ private static IRubyObject foreachInternal19(ThreadContext context, IRubyObject
                 }
             } finally {
                 io.close();
+                runtime.getGlobalVariables().clear("$_");
             }
         }